Since batteries are electrochemical devices, their performance gradually decreases over time. Premature wear-out means higher costs in terms of replacement labor and shorter service cycle. A worn battery entails a risk of unexpected load loss. In normal operation, the battery “wearing” rate depends strongly on how the full charge is being maintained. Excess charging is detrimental under any operating circumstances.
The Li-ion charger is a voltage-limiting device that is similar to the lead acid system. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge. While lead acid offers some flexibility in terms of voltage cut-off, manufacturers of Li-ion cells are very strict on the correct setting because Li-ion cannot accept overcharge. Li-ion is a “clean” system and only takes what it can absorb. Anything extra causes stress.
Most cells charge to 4.20V/cell with a tolerance of +/−50 mV/cell. Higher voltages could increase the capacity, but the resulting cell oxidation would reduce service life. More important is the safety concern if charging beyond 4.20V/cell.
Battery chargers for rechargeable batteries, especially for lithium batteries, generally have three charge stages. The first charge stage is a trigger charge stage. The second charge stage is a constant current model stage. The third charge stage is a constant voltage model stage. From the perspective of electronic technology for rechargeable batteries, the first charge stage is a limited current charge stage. The second charge stage is a high constant current stage. The third charge stage is a low constant voltage stage. Transition between the second charge stage and the third charge stage is determined by a charge current. When a current is larger than the charge current, it stays in the second charge stage; when the current is smaller than the charge current, it stays in the third charge stage. The charge current is also called switching current or transition current.
Please refer to FIG. 1. FIG. 1 shows a relation of current (I) and voltage (V) with time when the rechargeable battery is under charging. Variation of current is illustrated by a dashed line and variation of voltage is illustrated by a solid line. The first charge stage is from 0 to t1, the second charge stage is from t1 to t2, and the third charge stage is from t2 to t3. After t3, the rechargeable battery is no longer charged. In the first charge stage, the current keeps the same while the voltage increases in a roughly linear relationship with time. In the second charge stage, the current increases significantly and keeps at a constant value. Meanwhile, the voltage still increases but the rate decreases with time. When the rechargeable battery steps into the third charge stage, the current drops with time, and the voltage remains the same. The rechargeable battery is charged until a cut-off current met at t3. Since no current is inputted into the rechargeable battery after t3, the rechargeable battery will naturally decrease the voltage as well as the power stored inside.
Generally, chargers are set to have a cut-off point by the cut-off current mentioned above after t3 (completed time of the third charge stage) to protect the rechargeable battery from being over charged. It means that after the cut-off point on FIG. 1, no charge process is carried on. All rechargeable batteries have the same characteristics to discharge themself. If a rechargeable battery is under discharge situation for a long time without power supply (recharge), capacity of the rechargeable battery will decrease. As shown in FIG. 1, the voltage after t3 decreases. It causes not only low power capacity, but also low life time and low battery capacity.
Traditionally, a charging control system for lithium battery detects the current voltage of a charging lithium battery by a voltage detector and passes this voltage value to a microprocessor, which pre-exist in the hand-held apparatus. Thus, the microprocessor can decides the applicable charge stage and confirms the status of the charging battery depending on the different voltage values in real time. Next, the microprocessor controls a control unit by a pulse width modulation signal to modulate the power-source, which comes from an adaptor, as a constant current or a constant voltage to charge the battery. Accordingly, the charge process completed by repeat the voltage detection and the duty cycle modulation of the control unit. However, although the charging control system has convenient charging arrangement to get the rechargeable battery charged by detecting current power capacity, it still fails to control battery capacity if the rechargeable battery is fully charged.
Hence, a charging device for remaining a rechargeable battery in full capacity during standby after being fully charged is desired.